1. Introduction

Aim: Getting started with statistical approaches and bioinformatics tools commonly used to analyze microarray experiments and to select genes according to their expression profiles.

This practical is divided in 3 sections:

  1. Preprocessing of the raw data: correction of experimental biases
  2. Search for differentially expressed genes
  3. Functional analyzes of differentially expressed genes


The first microarray datasets were collected from the publication of Guida et al.2011. The authors used high throughput technologies (microarrays and high throughput sequencing) to determine the transcriptional profile of the pathogenic yeast Candida parapsilosis growing in several conditions including media, temperature and oxygen concentrations. You will use the datasets related to the study of the hypoxic (low oxygen) response in C. parapsilosis.

2. Preprocessing of the raw data: correction of experimental biases

The experiments were performed comparing one cell culture incubated at atmospheric oxygen conditions (call normoxic and labelled using Cy3 dye) and another one incubated in 1% O2 (call hypoxic and labelled using Cy5 dye).

Reading of GPR file

Input : A GPR file with detailed information for each spot on the slide (gene name, Cy5 and Cy3 intensity values, background intensities and other statistics).

Library and data loading

library(marray)

The R package Marray offers several functions to:

  • Read GPR files
  • Draw graphical representations of microarray results (foreground and background signals, missing values, MAplots, etc.)
  • Perform the normalization between Cy5 and Cy3 signals.
# Read the GPR file using the marray package function read.GenePix
rawdata <- read.GenePix(fnames="dataFile1_normAnalysis.gpr",
                        path= "/shared/projects/2420_ens_hts/data/microarrays/data")
## Reading ...  /shared/projects/2420_ens_hts/data/microarrays/data/dataFile1_normAnalysis.gpr

Note : This function reads a GPR file and creates objects of class marrayRaw. In these objects, you can find, for instance, vectors with intensity values (rawdata@maRf or rawdata@maGf). These vectors can be manipulated using classical R functions like “summary()”, “hist()”, etc.

Raw data manipulation

Take a few minutes to better understand the structure of the R object “marrayRaw”. Start for instance to manipulate the vectors with foreground signals (rawdata@maRf or rawdata@maGf).


# f is for foreground
# Intensity values in red/hypoxic channel
head(rawdata@maRf)
##      /shared/projects/2420_ens_hts/data/microarrays/data/dataFile1_normAnalysis.gpr
## [1,]                                                                            992
## [2,]                                                                           1907
## [3,]                                                                            559
## [4,]                                                                            645
## [5,]                                                                          32939
## [6,]                                                                            681
# Intensity values in green/normoxic channel
head(rawdata@maGf)
##      /shared/projects/2420_ens_hts/data/microarrays/data/dataFile1_normAnalysis.gpr
## [1,]                                                                           2561
## [2,]                                                                           2585
## [3,]                                                                           1588
## [4,]                                                                           1604
## [5,]                                                                          43755
## [6,]                                                                           1732

Visualisation of foreground signal

# Red/hypoxic signals
image(rawdata,
      xvar = "maRf",
      main = "Hypoxic signal (with flags)")

## [1] FALSE
## NULL
# Green/normoxic signals
image(rawdata,
      xvar = "maGf",
      main = "Normoxic signal (with flags)")

## [1] FALSE
## NULL

Evaluation of data quality

Visualisation of background signal

Visualize background signals in Red/Hypoxic and Green/Normoxic channels. Try to interpret the obtained results. How is the quality of the experiment?


# b is for background
# Red/Hypoxic channel background signals
image(rawdata,
      xvar = "maRb",
      main = "Hypoxic background (with flags)")

## [1] FALSE
## NULL
# Green/Normoxic channel background signals
image(rawdata,
      xvar = "maGb",
      main = "Normoxic background (with flags)")

## [1] FALSE
## NULL

Flags filtering

Each spot is associated with a flag value reporting some quality information. Flag values are the following:

  • -50 not found
  • -75 empty
  • -100 bad
  • 0 good
# Distribution of spots by flag values
table(rawdata@maW)
## 
## -100  -75  -50    0 
##  103  192 5922 9335

Negatively flagged spots will be eliminated from further analyses. For that, intensity values in foreground and background signals have to be replaced with the R symbol “NA” (missing values, “Not Available”).

# You work on a copy of the rawdata
rawdataWithoutFlags <- rawdata

#Background values to NA
rawdataWithoutFlags@maRb[rawdataWithoutFlags@maW < 0] = NA
rawdataWithoutFlags@maGb[rawdataWithoutFlags@maW < 0] = NA

#Signal values to NA
rawdataWithoutFlags@maRf[rawdataWithoutFlags@maW < 0] = NA 
rawdataWithoutFlags@maGf[rawdataWithoutFlags@maW < 0] = NA

Background correction

An intuitive approach for background correction consists in subtracting background intensity values (rawdata@maRb and rawdata@maGb) from global foreground intensities (rawdata@maRf and rawdata@maGf). Nevertheless this method is debatable because for low intensities it adds more noise to the data and creates overestimated log(Ratio) values.

#Draw a MA plot with background subtracted from foreground values 
plot(rawdataWithoutFlags,legend.func = NULL, main = "MA plot before normalization (background subtraction)")

#Replace all background by 0
rawdataWithoutFlags@maGb[] = 0
rawdataWithoutFlags@maRb[] = 0

#Draw a MA plot with foreground values only
plot(rawdataWithoutFlags,legend.func = NULL, main = "MA plot before normalization (no background subtraction)")

The following analyses will be performed with no background subtraction.

Normalization

Comparison of Cy5/Hypoxic and Cy3/Normoxic global signals

Draw the MA plot between Cy5/Hypoxic and Cy3/Normoxic signals.
  • Is there any experimental bias?
  • What information gives the boxplot representation?
  • What kind of normalization method needs to be applied? 


plot(rawdataWithoutFlags, main = "MA plot before normalization")

boxplot(rawdataWithoutFlags, main = "Boxplot before normalization")

Comparison of three different normalization procedures for signal normalization

Normalize the intensity measures between Cy5/Hypoxic and Cy3/Normoxic signals.
  • Try different normalization methods (“median”, “loess” and “printTipLoess”).
  • Draw the associated MA plot and BoxPlot (after normalization). What are the differences with the graphs obtained before normalization?
  • Draw the log2(R/G) distribution before and after normalization. How do you interpret the results? 


rawdataWithoutFlagsNorm <- maNorm(rawdataWithoutFlags, norm = "median", echo = T)
## Normalization method: median.
## Normalizing array 1.
rawdataWithoutFlagsNorm2 <- maNorm(rawdataWithoutFlags, norm = "loess", echo = T)
## Normalization method: loess.
## Normalizing array 1.
rawdataWithoutFlagsNorm3 <- maNorm(rawdataWithoutFlags, norm = "printTipLoess", echo = T)
## Normalization method: printTipLoess.
## Normalizing array 1.

Several plots allow for comparison of the normalization methods

plot(rawdataWithoutFlagsNorm, legend.func = NULL, main = "norm = Median")

plot(rawdataWithoutFlagsNorm2, legend.func = NULL, main = "norm = Loess")

plot(rawdataWithoutFlagsNorm3, legend.func = NULL, main = "norm = printTipLoess")

boxplot(rawdataWithoutFlagsNorm, main = "norm = Median")

boxplot(rawdataWithoutFlagsNorm2, main = "norm = Loess")

boxplot(rawdataWithoutFlagsNorm3, main = "norm = printTipLoess")

plot(density(maM(rawdataWithoutFlagsNorm2),na.rm = T),
     lwd = 2, col = 2, main = "Distribution of log(Ratio)")
lines(density(maM(rawdataWithoutFlags), na.rm = T), lwd = 2)
abline(v = 0)
legend(x= 0.5, y= 1.2,c("Before normalization","After normalization with loess"), fill = c(1,2))

3. Search for differentially expressed genes

In their article (Guida et al., 2011), the authors repeated the experiment 4 times for normoxic condition (with O2) and 4 times for hypoxic conditions (without O2). Expressions of genes between the two conditions were compared using microarrays (Ratio = hypoxia / normoxia).

You will perform the DE analysis using the limma package

Limma library loading

library(limma)

Data loading of the normalized log ratio intensity value for each replicates

Input: A text file with four different biological replicates (after normalization).

dataFile <- "/shared/projects/2420_ens_hts/data/microarrays/data/dataFile_diffAnalysis.txt"
data <- as.matrix(read.table(dataFile, row.names = 1, header = T))
# Retrieve some information from the data table
dim(data)
## [1] 5526    4
data[1:10,1:4]
##                   logVal1      logVal2      logVal3     logVal4
## CPAR2_201050 -0.265265616 -0.130465012  0.008997103 -0.06624613
## CPAR2_101960 -0.843512598 -0.608422137 -0.103000282 -0.45358870
## CPAR2_101290  0.056414092  0.000296908 -0.068354697  0.05983511
## CPAR2_405520  0.464588136  0.509999239  0.284349940  0.44530769
## CPAR2_201590 -0.230207648 -0.176294382 -0.265324830 -0.24833664
## CPAR2_103750 -0.194992750 -0.186335163  0.191242260 -0.57185971
## CPAR2_100170 -0.132982234 -0.191465175 -0.126354218  0.00331530
## CPAR2_202790  0.973402061  0.853915233  0.808972712  0.74969076
## CPAR2_301860 -0.008917937  0.018171339 -0.021780941  0.16899955
## CPAR2_106430 -1.598703129 -1.508676852 -0.642865880 -0.87494246
summary(data)
##     logVal1            logVal2             logVal3             logVal4         
##  Min.   :-3.20789   Min.   :-2.902426   Min.   :-3.634918   Min.   :-3.854502  
##  1st Qu.:-0.33699   1st Qu.:-0.318338   1st Qu.:-0.252692   1st Qu.:-0.278030  
##  Median :-0.01331   Median :-0.002482   Median : 0.002798   Median :-0.009852  
##  Mean   : 0.02278   Mean   : 0.025141   Mean   : 0.075240   Mean   : 0.014181  
##  3rd Qu.: 0.30528   3rd Qu.: 0.309444   3rd Qu.: 0.322220   3rd Qu.: 0.282426  
##  Max.   : 6.65491   Max.   : 6.422750   Max.   : 6.559013   Max.   : 6.213929

Linear model estimations and calculation of the Bayesian statistics

# Linear model estimation
fit <- lmFit(data)

# Bayesian statistics
limma.res <- eBayes(fit)
# Overview of the most differentially expressed genes
head(topTable(limma.res))
##                 logFC  AveExpr        t      P.Value    adj.P.Val        B
## CPAR2_404850 6.462651 6.462651 71.45643 3.788865e-10 2.093727e-06 13.59386
## CPAR2_503990 5.168192 5.168192 61.93992 9.047930e-10 2.499943e-06 13.02649
## CPAR2_502580 3.504953 3.504953 50.62005 3.091002e-09 4.333583e-06 12.10863
## CPAR2_807620 3.614666 3.614666 50.49767 3.136868e-09 4.333583e-06 12.09684
## CPAR2_401230 3.328905 3.328905 45.27100 6.097772e-09 5.982882e-06 11.54690
## CPAG_00607   3.396506 3.396506 43.79661 7.457963e-09 5.982882e-06 11.37366
# Retrieve result table for all genes
allgenes.limma <- topTable(limma.res, number = nrow(data))
Identify the differentially expressed genes using LIMMA method.
  • Try different adjusted p-value thresholds: 5%, 1%, etc..
  • How many genes are induced in hypoxic condition (without 02  with 02)?
  • How many genes are repressed in hypoxic condition (without O2  with 02)?


Plot results of the differential analysis

You will draw a Volcano plot using the EnhanceVolcano Bioconductor package

EnhanceVolcano library loading

library(EnhancedVolcano)

Threshold selection

logFCthreshold <- 2
adjPVthreshold <- 0.01

Volcano plot drawing

EnhancedVolcano(allgenes.limma,
    lab = rownames(allgenes.limma),
    x = 'logFC',
    y = 'adj.P.Val',
    axisLabSize = 12,
    ylim = c(0,max(-log10(allgenes.limma[[5]]))),
    caption = paste0(nrow(allgenes.limma), " total genes"),
    captionLabSize = 12,
    FCcutoff = logFCthreshold,
    pCutoff = adjPVthreshold,
    title = 'Hypoxia VS Normoxia',
    subtitle = 'Volcano plot',
    pointSize = 2.0,
    labSize = 4.0,
    legendLabSize = 12,
    legendIconSize = 2.0,
    drawConnectors = TRUE)
## Warning: ggrepel: 58 unlabeled data points (too many overlaps). Consider
## increasing max.overlaps

You can use the option “selectLab” to display only the genes you are interested in on the graph passing a vector of gene names.

EnhancedVolcano(allgenes.limma,
    lab = rownames(allgenes.limma),
    x = 'logFC',
    y = 'adj.P.Val',
    selectLab = c('CPAR2_404850','CPAR2_602990', 'CPAR2_802720'),
    axisLabSize = 12,
    ylim = c(0,max(-log10(allgenes.limma[[5]]))),
    caption = paste0(nrow(allgenes.limma), " total genes"),
    captionLabSize = 12,
    FCcutoff = logFCthreshold,
    pCutoff = adjPVthreshold,
    title = 'Hypoxia VS Normoxia',
    subtitle = 'Volcano plot',
    pointSize = 2.0,
    labSize = 4.0,
    legendLabSize = 12,
    legendIconSize = 2.0,
    drawConnectors = TRUE)

Selection of differentially expressed genes (using previous thresholds)

# Filter differentially expressed genes based on previous thresholds
siggenes.limma <- allgenes.limma[allgenes.limma[,5] < adjPVthreshold & abs(allgenes.limma[,1]) > logFCthreshold,] 

paste(dim(siggenes.limma[siggenes.limma[,1] > 0,])[1], "upregulated genes in Hypoxia")
## [1] "78 upregulated genes in Hypoxia"
paste(dim(siggenes.limma[siggenes.limma[,1] < 0,])[1], "upregulated genes in Normoxia")
## [1] "11 upregulated genes in Normoxia"
# Export DE gene table into your home directory:
write.table(siggenes.limma[siggenes.limma[,1] > 0,], 
            row.names = T, quote = F, sep = ";",
            file = "~/hypoxia_signif_genes.csv")

write.table(siggenes.limma[siggenes.limma[,1] < 0,], 
            row.names = T, quote = F, sep = ";",
            file = "~/normoxia_signif_genes.csv")

print(siggenes.limma[siggenes.limma[,1] < 0,])
##                  logFC   AveExpr          t      P.Value    adj.P.Val         B
## CPAR2_602990 -2.857353 -2.857353 -40.921642 1.126926e-08 6.227391e-06 11.009444
## CPAR2_603010 -2.386064 -2.386064 -36.666590 2.195952e-08 7.238432e-06 10.396967
## CPAR2_700940 -2.716475 -2.716475 -33.338684 3.912918e-08 9.828540e-06  9.845186
## CPAR2_808020 -2.107592 -2.107592 -26.385586 1.614952e-07 2.059145e-05  8.421440
## CPAR2_108370 -3.399933 -3.399933 -21.054322 6.312015e-07 4.166148e-05  6.981216
## CPAR2_207540 -2.456284 -2.456284 -21.027727 6.360268e-07 4.166148e-05  6.973018
## CPAR2_300630 -2.913243 -2.913243 -20.849107 6.695692e-07 4.302371e-05  6.917658
## CPAR2_603050 -2.369657 -2.369657 -18.816323 1.241135e-06 6.178842e-05  6.248014
## CPAR2_802720 -2.030915 -2.030915 -13.789453 7.945638e-06 2.080929e-04  4.191489
## CPAR2_800530 -2.756817 -2.756817 -11.269853 2.610170e-05 4.397499e-04  2.852306
## CPAR2_802670 -2.383530 -2.383530  -8.602067 1.243087e-04 1.152567e-03  1.082295

4. Functional analyzes of differentially expressed genes

Several tools are available online to evaluate the biological relevance of the gene sets you select after the differential analysis. For example you can you use the GoTermFinder tool dedicated to Candida yeast species to retrieve functional annotation. Be careful to choose the genome of Candida parapsilosis in the species selection dialog box. You can also obtain information on a specific gene using the Candida Genome Database.

What are the functions of the genes in your lists (identified at the previous step).
  • Are they relevant with the studied biological system described by Guida et al. in their publication?


Reproductibility

#Date
format(Sys.time(), "%d %B, %Y, %H,%M")
## [1] "20 August, 2024, 13,53"
#Packages used
sessionInfo()
## R version 4.4.1 (2024-06-14)
## Platform: x86_64-conda-linux-gnu
## Running under: Ubuntu 20.04.6 LTS
## 
## Matrix products: default
## BLAS/LAPACK: /shared/ifbstor1/software/miniconda/envs/r-4.4.1/lib/libopenblasp-r0.3.27.so;  LAPACK version 3.12.0
## 
## locale:
##  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
##  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
##  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
##  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
##  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
## [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
## 
## time zone: Europe/Paris
## tzcode source: system (glibc)
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] EnhancedVolcano_1.22.0 ggrepel_0.9.5          ggplot2_3.5.1         
## [4] marray_1.82.0          limma_3.60.3          
## 
## loaded via a namespace (and not attached):
##  [1] gtable_0.3.5      jsonlite_1.8.8    dplyr_1.1.4       compiler_4.4.1   
##  [5] highr_0.11        tidyselect_1.2.1  Rcpp_1.0.12       jquerylib_0.1.4  
##  [9] scales_1.3.0      yaml_2.3.9        fastmap_1.2.0     statmod_1.5.0    
## [13] R6_2.5.1          labeling_0.4.3    generics_0.1.3    knitr_1.48       
## [17] tibble_3.2.1      munsell_0.5.1     bslib_0.7.0       pillar_1.9.0     
## [21] rlang_1.1.4       utf8_1.2.4        cachem_1.1.0      xfun_0.45        
## [25] sass_0.4.9        cli_3.6.3         withr_3.0.0       magrittr_2.0.3   
## [29] digest_0.6.36     grid_4.4.1        rstudioapi_0.16.0 lifecycle_1.0.4  
## [33] vctrs_0.6.5       evaluate_0.24.0   glue_1.7.0        farver_2.1.2     
## [37] fansi_1.0.6       colorspace_2.1-0  rmarkdown_2.27    tools_4.4.1      
## [41] pkgconfig_2.0.3   htmltools_0.5.8.1